Heretofore, a thin liquid crystal display uses a backlight that illuminates a liquid crystal panel from the back toward the front face of the liquid crystal panel, and is broadly classified into an edge lighting system and a direct lighting system depending on structures of the backlight.
In the edge lighting system, incident light from the side surface of a light guide plate is propagated to the inside of the light guide plate, and then, the light is extracted from the upper surface of the light guide plate. On the other hand, in the direct lighting system, for example, a plurality of fluorescent tube lamps, such as, cold cathode fluorescent lamps (CCFLs) or the like are provided on a substrate to perform surface emission on the whole surface (see Patent Literature 1).
In recent years, from the viewpoints of increasing the size, decreasing the thickness, decreasing the weight, extending the lifetime of the liquid crystal display, and improving moving image properties by controlling flashing, there has been employed the direct lighting system in which a plurality of light-emitting diodes (LEDs) are provided on a substrate to perform surface emission.
A first method using the LED is a method involving providing LEDs emitting colored light of three colors (R, G, and B) and turning on the LEDs simultaneously to combine the light of three colors to generate white light.
Furthermore, a second method using the LED is a method involving surrounding, for example, a blue light-emitting LED chip with resin containing phosphor particles so as to convert blue light into white light.
Moreover, a third method using the LED is a method of which light from a blue light-emitting diode (blue LED) is irradiated to a phosphor sheet obtained by dispersing a powdery red phosphor particle that emits red fluorescence when irradiated with blue light and a green phosphor particle that emits green fluorescence when irradiated with blue light in a resin material having a excellent property in visible light transmittance to emit red light and green light; and the red light and green light with blue light are mixed to generate white light (see Patent Literatures 2 and 3).
Some phosphor particles are fragile against oxygen or water vapor. When the phosphor particles are exposed to oxygen or water vapor, their properties deteriorate to cause luminance or chromaticity unevenness. For example, a sulfide phosphor particle easily deteriorates by an environment, such as, water or oxygen and significantly deteriorates particularly under high-temperature and high-humidity conditions. Deterioration of the sulfide phosphor particle reduces the lifetime of an LED element because a sulfur component in the phosphor particles itself causes corrosion of a metal at a current-carrying portion in the LED element to induce a decrease in light extraction efficiency in the LED element, break of an energization portion, and the like.
In order to extend the lifetime of an LED element using a sulfide phosphor particle, there have been proposed a method involving forming a protective layer formed of a silicon compound or the like on the phosphor particle (see Patent Literature 4), a method involving adding an adsorbent for a sulfur-based gas to a resin for sealing an LED (see Patent Literature 5), and a method involving performing sealing with an oxygen or water-vapor barrier film to extend its lifetime (see Patent Literatures 6 and 7).
However, even when the surface of the phosphor particle is coated as described in Patent Literature 4, chromaticity shift or corrosion due to deterioration of the sulfide phosphor particle cannot be suppressed sufficiently. Furthermore, in the technologies of Patent Literatures 5 to 7, the phosphor particle itself is not protected, and deterioration of the phosphor particle during long-term use is unavoidable.